A semiconductor is generally fabricated by projecting a pattern formed in a photomask (hereinafter, also referred to as a mask) on a wafer onto which a resist is applied. That is, the mask assumes the role of a negative in a camera. If the mask has a defect, the defect will be transferred to all wafers fabricated by using the mask. Thus, a defect inspection of mask plays an extremely important role in a semiconductor fabrication process.
A method of pattern-to-pattern comparison on a mask (die-to-die) and that of inspection using design data of a mask (die-to-database) are known as methods of the defect inspection of mask. Both these inspection methods cannot inspect a mask region without pattern information. Actually, however, it is necessary to inspect for defects, among others, foreign particle also in a region without pattern information such as a scribe area of mask.
Methods described, for example, in JP-A H08-76359 (KOKAI) are well known as inspection methods of a mask region without pattern information. These methods include, for example, as shown in FIG. 13, a method by which a region 91 within which a relationship between a reflected image (horizontal axis R in FIG. 13) and a transmitted image (vertical axis T in FIG. 13) of each point where there is no mask defect always stays is set and points deviating from the region 91 are determined as defects. This method is effective under ideal conditions under which there is no relative position shift between a reflected image and a transmitted image and there is almost no noise. Actually, however, there are cases of deviation from the region 91 without defect or incorporation of defect into the region 91 due to an influence of a relative position shift between a reflected image and a transmitted image or noise. This is a phenomenon frequently observed particularly in pattern edges. As countermeasures to prevent a relative position shift between a reflected image and a transmitted image, as shown in FIG. 14, a method by which a region 92 within which a relationship between a reflected image (horizontal axis R in FIG. 14) and a second derivative image (vertical axis R″ in FIG. 14) thereof always stays is set and points deviating from the region 92 are determined as defects and, as shown in FIG. 15, a method by which a region 93 within which a relationship between a transmitted image (horizontal axis T in FIG. 15) and a second derivative image (vertical axis T″ in FIG. 15) thereof always stays is set and points deviating from the region 93 are determined as defects are also proposed in JP-A H08-76359 (KOKAI).
Indeed, even the methods by JP-A H08-76359 (KOKAI) do not take sufficient steps against erroneous identification due to an influence of noise.